The present invention in the first place relates to a squeegee assembly in particular intended for coating substantially cylindrical objects with a liquid or pasty material, at least comprising an annular squeegee with an outer mounting edge and an inner levelling edge.
Such a squeegee assembly is known from the prior art and is frequently used in coating devices and also manually in specially designed manual coating devices, in order to provide substantially cylindrical objects with a coating of material. Such a squeegee assembly is for example used for coating a seamless metal rotary screen printing cylinder, which is, for example, a nickel cylinder having a wall thickness of between 50-500 microns and comprising 20-200 holes per running cm, with a lacquer. After such a screen printing cylinder has been coated, that is to say the lacquer coating on the dams between the holes has a specific layer thickness and a suitable depth of penetration into the holes, said lacquer coating is partially removed in accordance with a pattern, while the remaining areas are hardened. Such a screen printing cylinder is then ready for screen printing, in which case a dye is pressed on a substrate to be printed through the open holes by means of an internal straight squeegee, as known from the prior art.
For the purpose of clarity in the following description reference will be made exclusively to the coating of a rotary screen printing cylinder with a lacquer. The device may, however, also be used for a large number of other applications, such as for example, painting or otherwise coating substantially cylindrical objects, like tubes or even substantially solid objects.
The coating of cylindrical objects with the aid of known squeegee assemblies has a number of important disadvantages. The accuracy with which such squeegee assembly has to be displaced along the object to be coated is required to be very high, as substantial differences in layer thickness and penetration depth of the material to be applied occur in the case of even the smallest eccentricity between the squeegee and the object. Furthermore the choice of the shape of the squeegee and its rigidity as well as the choice of the lacquer are relatively complicated and dependent on many factors. The layer thickness and penetration depth of the material to be applied to a screen printing cylinder are dependent on the rigidity of the squeegee, the speed of displacement of the squeegee and the properties of the lacquer, such as viscosity, solids content, surface tension etc. Up till now it has not been possible to apply relatively smooth thick coatings of lacquer in one coating run, on the object and/or sufficient penetration depth into the holes of a screen; mainly avoiding the setting effect above the holes.
Further different squeegees are required for upward-respectively downward coating. During upward coating a relatively stiff squeegee is preferred, whereas with downward coating a more flexible squeegee is required. In case of a relatively stiff squeegee the squeegee assembly displacement means have to be designed in such a manner, that a very accurate displacement of the squeegee with respect to the cylinder to be coated it guaranteed. Moreover, the positioning of the screen printing cylinder in the coating device also has to be carried out very accurately. Even with very small deviations in the centring of the displacement of the squeegee over the cylinder, differences in layer thickness, streaks and even leakage phenomena are immediately observable.
During coating in the downward direction the more flexible squeegee can absorb greater variation in the centring of the squeegee and the cylinder, but in this case there is a greater dependency on the choice of the lacquer. The fact is that this coating method is largely dependent on the lacquer properties, together with the downward speed of movement of the squeegee. Finally this coating method is relatively slow (varying from 5-15 cm/min).
The squeegee assembly may comprise only an optionally clamped squeegee, but generally will also comprise a mounting block or e.g. a bowl part, which is used to mount the assembly in a coating device. Together with the squeegee and the cylinder surface to be coated during the coating operation, said mounting block or bowl part forms a reservoir which can generally contain substantially more than the amount of lacquer required